Front-wheel drive is a form of engine and transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel-drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement found in rear-wheel-drive and four-wheel drive vehicles. By far the most common layout for a front-wheel drive car is a with the engine and transmission at the front of the car, mounted transversely. Other layouts of front-wheel drive that have been produced are a front-engine mounted longitudinally, a mid-engine layout and a rear-engine layout. Experiments with front-wheel-drive cars date to the early days of the automobile. According to various sources, sometime between 1895 and 1898 Gräf & Stift built a voiturette with a one-cylinder De Dion-Bouton engine fitted in the front of the vehicle, powering the front axle, it was thus arguably the world's first front-wheel-drive automobile, but it never saw mass production, with only one copy made.
In 1898, Latil, in France, devised a front-wheel-drive system for motorising horse-drawn carts. In 1898/9 the French manufacturer Société Parisienne patented their front-wheel drive articulated vehicle concept which they manufactured as a Victoria Combination, it was variously powered by 1.75 or 2.5 horsepower De Dion-Bouton engine or a water cooled 3.5 horsepower Aster engine. The engine so was rotated by the tiller steering; the name Victoria Combination described the lightweight, two-seater trailer known as a Victoria, combined with the rear axle and drive mechanism from a motor tricycle, placed in front to achieve front wheel drive. It known as the Eureka. By 1899 Victoria Combinations were participating in motoring events such as the 371 km Paris-St Malo race, finishing 23rd overall and second in the class. In October a Victoria Combination won its class in the Paris-Rambouillet-Paris event, covering the 100 kilometre course at 26 km/h. In 1900 it completed 240 kilometres non-stop at 29 km/h.
When production ceased in mid-1901, over 400 copies had been sold for 3,000 Francs each. J. Walter Christie of the United States patented a design for a front-wheel-drive car, the first prototype of which he built in 1904, he promoted and demonstrated the vehicle by racing at various speedways in the United States, competed in the 1906 Vanderbilt Cup and the French Grand Prix. In 1912 he began manufacturing a line of wheeled fire engine tractors which used his front-wheel-drive system, but due to lack of sales this venture failed; the next successful application of front-wheel drive was the supercharged Alvis 12/50 racing car designed by George Thomas Smith-Clarke and William M. Dunn of Alvis Cars of the United Kingdom; this vehicle was entered in the 1925 Kop Hill Climb in Princes Risborough in Buckinghamshire on March 28, 1925. Harry Arminius Miller of Menomonie, Wisconsin designed the Miller 122 front-wheel-drive racecar, entered in the 1925 Indianapolis 500, held at the Indianapolis Motor Speedway on Saturday, May 30, 1925.
However, the idea of front-wheel drive languished outside the motor racing arena as no major auto manufacturer attempted the same for production automobiles. Market experiments in the United States were left to small endeavors such as the Ruxton, the Cord L-29 of 1929. Neither automobile maker was successful on the open market. Alvis Cars introduced a front-wheel-drive commercial model of the Alvis 12/50 in 1928, but it was not a success either; the first successful consumer application came in 1929. The BSA produced the unique front-wheel-drive BSA three-wheeler. Production continued until 1936. In 1931 the DKW F1 from Germany made its debut. Buckminster Fuller adopted front wheel drive for his three Dymaxion Car prototypes. Other German car producers followed: Stoewer offered a car with front-wheel drive in 1931, Adler in 1932 and Audi in 1933. In 1934, the successful Traction Avant cars were introduced by Citroën of France; the Cord 810 of the United States managed a bit better in the late 1930s than its predecessor one decade earlier.
These vehicles featured a layout that places the engine behind the transmission, running "backwards,". The basic front-wheel-drive layout provides sharp turning, better weight distribution creates "positive handling characteristics" due to its low polar inertia and favourable weight distribution.. Another result of this design is a lengthened chassis. After the 1930s, front-wheel drive would become abandoned for the following twenty years. Front-wheel drive continued with the 1948 Citroën 2CV, where the air-cooled lightweight aluminium flat twin engine was mounted ahead of the front wheels, but used Hooke type universal joint driveshaft joints, 1955 Citroën DS, featuring the mid-engine layout. Panhard of France, DKW of Germany and Saab of Sweden offered front-wheel-drive cars, starting with the 1948 Saab 92. In 1946, Lloyd Cars, the English car company, had produced the front-wheel-drive roadster, Lloyd 650; the two-stroke, two-cylinder motor was mounted transversely in the front and connected to the front wheels through four-speed synchronised gearbox.
The high price and lacklustre performance had doomed its production. Only 600 units were produced from 1946 to 1950. In 1954, Alfa-Romeo had experimented with its first front-wheel-drive compact car named "33" (not related or ref
British Leyland was an automotive engineering and manufacturing conglomerate formed in the United Kingdom in 1968 as British Leyland Motor Corporation Ltd, following the merger of Leyland Motors and British Motor Holdings. It was nationalised in 1975, when the UK government created a holding company called British Leyland BL, in 1978, it incorporated much of the British-owned motor vehicle industry, which constituted 40 percent of the UK car market, with roots going back to 1895. Despite containing profitable marques such as Jaguar and Land Rover, as well as the best-selling Mini, British Leyland had a troubled history, leading to its eventual collapse in 1975 and subsequent nationalisation. After much restructuring and divestment of subsidiary companies, it was renamed as the Rover Group in 1986 becoming a subsidiary of British Aerospace and subsequently, BMW; the final surviving incarnation of the company as the MG Rover Group, which went into administration in 2005, bringing mass car production by British-owned manufacturers to an end.
MG and the Austin and Wolseley marques became part of China's SAIC, with whom MG Rover attempted to merge prior to administration. Today, Jaguar Land Rover and Leyland Trucks are the three most prominent former parts of British Leyland which are still active in the automotive industry, with SAIC-owned MG Motor continuing a small presence at the Longbridge site. Certain other related ex-BL businesses, such as Unipart, continue to operate independently. BLMC was created on 17 January 1968 by the merger of British Motor Holdings and Leyland Motor Corporation, encouraged by Tony Benn as chairman of the Industrial Reorganisation Committee created by the first Wilson Government. At the time, LMC was a successful manufacturer; the Government was hopeful LMC's expertise would revive the ailing BMH, create a "British General Motors". The merger combined most of the remaining independent British car manufacturing companies and included car and truck manufacturers and more diverse enterprises including construction equipment, metal casting companies, road surface manufacturers.
The new corporation was arranged into seven divisions under Sir Donald Stokes. While BMH was the UK's largest car manufacturer, it offered a range of dated vehicles, including the Morris Minor, introduced in 1948 and the Austin Cambridge and Morris Oxford, which dated back to 1959. Although BMH had enjoyed great success in the 1960s with both the Mini and the 1100/1300, both cars were infamously underpriced and despite their pioneering but unproven front wheel drive engineering, warranty costs had been crippling and had badly eroded those models' profitability. After the merger, Lord Stokes was horrified to find that BMH had no plans to replace the elderly designs in its portfolio. BMH's design efforts prior to the merger had focused on unfortunate niche market models such as the Austin Maxi and the Austin 3 litre, a car with no discernible place in the market; the lack of attention to the development of new mass-market models meant that BMH had nothing in the way of new models in the pipeline to compete with popular rivals such as Ford's Escort and Cortina.
Lord Stokes instigated plans to design and introduce new models quickly. The first result of this crash programme was the Morris Marina in early 1971, it used parts from various BL models with new bodywork to produce BL's mass-market competitor. It was one of the strongest-selling cars in Britain during the 1970s, although by the end of production in 1980 it was regarded as a dismal product that had damaged the company's reputation; the Austin Allegro, launched in 1973, earned a unwanted reputation over its 10-year production life. The company became an infamous monument to the industrial turmoil. Industrial action instigated by militant shop stewards brought BL's manufacturing capability to its knees. Despite the duplication of production facilities as a result of the merger, there were multiple single points of failure in the company's production network which meant that a strike in a single plant could stop many of the others. Both Ford and General Motors had mitigated against this years before by merging their separate British and German subsidiaries and product lines, so that production could be sourced from either British or Continental European plants in the event of industrial unrest.
The upshot was that both Ford and Vauxhall overtook BL to become Britain's two best selling marques, a title they hold to the present day. At the same time, a tide of Japanese imports, spearheaded by Nissan and Toyota exploited both BL's inability to supply its customers and its declining reputation for quality – by the end of the 1970s, the British government had introduced protectionist measures in the form of import quotas on Japanese manufacturers in order to protect the ailing domestic producers, which it was helping to sustain. At its peak, BLMC owned 40 manufacturing plants across the country. Before the merger BMH had included theoretically competing marques that were
BL O-series engine
The BL O-series engine is a straight-4 automobile engine family, produced by the Austin-Morris division of British Leyland as a development of the BMC B-series engine family. Introduced by BL in 1978 in the rear wheel drive Series 3 Morris Marina and the smaller engined versions of the front-wheel-drive Princess, it was intended to replace the 1.8 L B-series unit. The main advance over the B series was that the new unit was of belt driven overhead camshaft configuration, with an aluminium cylinder head. Offered in the unusual capacity of 1.7 L as well as 2.0 L, it proved to be reliable and was used in BL vehicles. These included the rear wheel drive Morris Ital of 1980, the rear wheel drive Rover SD1 of 1982, 1.7 L and 2.0 L in the front wheel drive Austin Ambassador – in fact the only engine offered in this model. In 1984, it was reworked for installation in high specification 2.0 L versions of the front wheel drive Austin Maestro and Austin Montego, where it was optionally available with fuel injection or turbo-charging.
This installation of the O series was adapted for use with the Honda PG-1 end-on manual gearbox, replacing the gearbox-in-sump design traditionally used on British Leyland front-wheel-drive products. The 1.7 L O series was not used in these vehicles, which featured R- and S-series 1.6 L units. The cylinder blocks of the transverse and earlier longitudinally mounted versions of the engine are not however interchangeable owing to differences in the gearbox mounting flange. In 1986, BL collaborated with Perkins to convert the O series to run on diesel; the oil-burning versions, known as the Rover MDi or Perkins Prima, proved to be successful in the Maestro and Montego, helped sustain the ailing mid-sized models into the 1990s. Perkins marketed the engine under its own brand in the industrial and marine sectors. By 1987, British Leyland equipped the O series with a 16-valve cylinder head for the Rover 800; this 2.0 L unit was known as the M series, was further reworked into the T series in 1992. The original 8-valve version of the O series was briefly used in budget versions of the Rover 800.
A notable advantage of this particular O-series engine is that the cylinder head does not require modification to run on unleaded petrol due to having hardened valve seats. Other O-series engines, cannot run on unleaded without modification of the cylinder head or use of an additive. Examples of vehicles using a version of the O-series engine: Austin Maestro 2.0 L, 2.0 L Diesel Austin Montego 2.0 L, 2.0 L Diesel Leyland Sherpa/Freight Rover Sherpa/200/300 1.7 L, 2.0 l Princess / Austin Ambassador 1.7 L, 2.0 L Morris Ital 1.7 L, 2.0 L automatic Morris Marina 1.7 L Rover SD1 2.0 L Austin Maestro 2.0 L Austin Montego 2.0 L Rover 820 2.0 L MG Maestro 2.0 L MG Maestro 2.0 L, turbo MG Montego 2.0 L MG Montego 2.0 L, turbo Naylor/Hutson TF 1700 O-series engine story
Internal combustion engine
An internal combustion engine is a heat engine where the combustion of a fuel occurs with an oxidizer in a combustion chamber, an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine; the force is applied to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy; the first commercially successful internal combustion engine was created by Étienne Lenoir around 1859 and the first modern internal combustion engine was created in 1876 by Nikolaus Otto. The term internal combustion engine refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as described.
Firearms are a form of internal combustion engine. In contrast, in external combustion engines, such as steam or Stirling engines, energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or liquid sodium, heated in a boiler. ICEs are powered by energy-dense fuels such as gasoline or diesel fuel, liquids derived from fossil fuels. While there are many stationary applications, most ICEs are used in mobile applications and are the dominant power supply for vehicles such as cars and boats. An ICE is fed with fossil fuels like natural gas or petroleum products such as gasoline, diesel fuel or fuel oil. There is a growing usage of renewable fuels like biodiesel for CI engines and bioethanol or methanol for SI engines. Hydrogen is sometimes used, can be obtained from either fossil fuels or renewable energy. Various scientists and engineers contributed to the development of internal combustion engines.
In 1791, John Barber developed the gas turbine. In 1794 Thomas Mead patented a gas engine. In 1794, Robert Street patented an internal combustion engine, the first to use liquid fuel, built an engine around that time. In 1798, John Stevens built the first American internal combustion engine. In 1807, French engineers Nicéphore and Claude Niépce ran a prototype internal combustion engine, using controlled dust explosions, the Pyréolophore; this engine powered a boat on France. The same year, the Swiss engineer François Isaac de Rivaz built an internal combustion engine ignited by an electric spark. In 1823, Samuel Brown patented the first internal combustion engine to be applied industrially. In 1854 in the UK, the Italian inventors Eugenio Barsanti and Felice Matteucci tried to patent "Obtaining motive power by the explosion of gases", although the application did not progress to the granted stage. In 1860, Belgian Jean Joseph Etienne Lenoir produced a gas-fired internal combustion engine. In 1864, Nikolaus Otto patented the first atmospheric gas engine.
In 1872, American George Brayton invented the first commercial liquid-fuelled internal combustion engine. In 1876, Nikolaus Otto, working with Gottlieb Daimler and Wilhelm Maybach, patented the compressed charge, four-cycle engine. In 1879, Karl Benz patented a reliable two-stroke gasoline engine. In 1886, Karl Benz began the first commercial production of motor vehicles with the internal combustion engine. In 1892, Rudolf Diesel developed compression ignition engine. In 1926, Robert Goddard launched the first liquid-fueled rocket. In 1939, the Heinkel He 178 became the world's first jet aircraft. At one time, the word engine meant any piece of machinery—a sense that persists in expressions such as siege engine. A "motor" is any machine. Traditionally, electric motors are not referred to as "engines". In boating an internal combustion engine, installed in the hull is referred to as an engine, but the engines that sit on the transom are referred to as motors. Reciprocating piston engines are by far the most common power source for land and water vehicles, including automobiles, ships and to a lesser extent, locomotives.
Rotary engines of the Wankel design are used in some automobiles and motorcycles. Where high power-to-weight ratios are required, internal combustion engines appear in the form of combustion turbines or Wankel engines. Powered aircraft uses an ICE which may be a reciprocating engine. Airplanes can instead use jet engines and helicopters can instead employ turboshafts. In addition to providing propulsion, airliners may employ a separate ICE as an auxiliary power unit. Wankel engines are fitted to many unmanned aerial vehicles. ICEs drive some of the large electric generators, they are found in the form of combustion turbines in combined cycle power plants with a typical electrical output in the range of 100 MW to 1 GW. The high temperature exhaust is used to superheat water to run a steam turbine. Thus, the efficiency is higher because more energy is extracted from the fuel than what could be extracted by the co
The Austin Maestro is a five-door hatchback small family car, produced from 1982 to 1987 by British Leyland, from 1988 until 1994 by Rover Group. The car was produced at Morris' former Oxford plant known as Cowley. Today, the redeveloped factory produces the BMW Mini. An MG-branded performance version was sold as the MG Maestro from 1983 until 1991. Although models were sometimes referred to as the Rover Maestro, the model never wore the Rover badge. A three-box car, the Montego, was a derivative of the Maestro. British Leyland was created in 1975 when the bankrupt British Leyland Motor Corporation was nationalised. In 1977 the South African-born corporate troubleshooter, Michael Edwardes, was recruited as chairman to sort out the troubled firm. Part of Edwardes' plan was to introduce a new range of mass-market models to replace the current offerings and built using state-of-the-art technology; the new range decided upon consisted of a new vehicle for each of the small, lower-medium and upper-medium market segments.
The first of these cars to be launched was the Austin Metro in 1980. The new cars for the lower and upper medium segments were to share a platform, with various trim and styling differences to distinguish the two different models; the two models would in effect replace four existing vehicles in the British Leyland range – the Maestro would replace both the Austin Allegro and Maxi, whilst the Montego replaced the Austin Ambassador and Morris Ital, these latter two having been recent facelifts of the Princess and Morris Marina. Since all but the Allegro were made at the Cowley plant, this rationalization would give the cost benefits of production automation and flexibility; this common platform was given the project name LC10. Preliminary design work for LC10 began in 1977, with production scheduled to begin around 1980 - which would have seen it go on sale around the same time as the Ford Escort MK3 and the original Vauxhall Astra. LC10 was styled by Ian Beech under the direction of BL designer David Bache.
Two main body variations were provided: a five-door hatchback and a four-door notchback. It was a departure from previous front-wheel drive cars from the company in dispensing with the famous Issigonis transmission-in-sump powertrain, pioneered in the Mini. Coupled to the A- and R-series powerplants was an end-on transmission, bought from Volkswagen; the sophisticated Hydragas suspension system used on previous BL models was sacrificed on cost grounds, with a conventional MacPherson strut system at the front and a Volkswagen Golf-style torsion beam at the rear being used instead - but with long travel rising rate springs. While easier to build, this suspension did compromise load space. Prototypes were tested with actual Golf suspension components; this may have led to the early cars being prone to front wheel bearing issues. The Maestro was heavier than the first VW Golf, it was decided. It was given its own project designation, LM10, with this version to be launched as the Austin Maestro; the name "Maestro" had been a finalist when the Austin Metro was being named, with the third choice never picked up.
The booted notchback version was to follow and it was designated as LM11, although its development was to diverge from the original path, it was launched as the Austin Montego on its launch in April 1984, following British Leyland's decision to discontinue the Morris marque. Production began in November 1982, the car was launched on 1 March 1983; the wheelbase was 2,510 mm, the length was 4,050 mm. The Maestro incorporated many novel and pioneering features for its class, it had a bonded laminated windscreen, homofocal headlamps, body-coloured plastic bumpers, an electronic engine management system, adjustable front seat belt upper anchorage positions, an asymmetrically split rear seat, a 12,000-mile service interval. The MG and Vanden Plas versions had solid-state instrumentation with digital speedometer and vacuum fluorescent analogue displays for tachometer and temperature gauges, trip computer and a voice synthesis warning and information system; the Maestro was launched in March 1983. In its summing up of the new car the Consumers' Association, in the June edition of its Which? journal, described it as roomy and nice to drive, said "If you are considering buying one now, our advice, based on our first impressions, is to go ahead".
In January 1984, after testing the car, they concluded: "In comparison with opposition of a similar price and body size, the Maestro has a clear advantage on room for passengers, with few cars equalling it for comfort either in the front or back". They considered it to be a serious rival to the higher-segment Vauxhall Cavalier and Ford Sierra, apart from its smaller boot space; the original lineup consisted of the 1.3-liter base, L, HLE models, the 1.6-liter L, HLS, Vanden Plas, the sporty MG Maestro. The HLE model had a somewhat downtuned engine and received Volkswagen's "monstrously long-geared" 3+E transmission to maximize fuel economy, at the cost of severe performance loss. To further up the HLE's economy game, it was fitted with an econometer and the same black rubber fins along the sides of the rear windshield as was the MG Maestro; the base model forewent the other versions' plastic bumpers, instead being fitted with black-painted steel units. The plastic bumpers were the first of their kind, being made fr
Austin Rover Group
The Austin Rover Group was a British motor manufacturer. It was created in 1982 as the mass-market car manufacturing subsidiary of British Leyland; this entity had been known as BL Cars Ltd which encompassed the Austin-Morris and Jaguar-Rover-Triumph divisions of British Leyland. After a major restructuring of BL's car manufacturing operations, Jaguar regained its independence whilst the Triumph and Morris marques were retired; the new, leaner car business was rechristened as the Austin Rover Group and focused on the Austin and Rover marques. The Morris and Triumph marques continued within ARG until 1984 when both were dropped. In 1989, two years after the Austin brand was discontinued, ARG assumed the name of its parent company Rover Group plc, from which point the two entities were considered one and the same, although they continued to be separate - Rover Group plc was a holding company owned Land Rover following the divestment of Unipart and Leyland Trucks, whilst Rover Group Limited was the mass market car manufacturing business.
Following the financial collapse of the British Leyland Motor Corporation in 1975 and the stark Ryder Report on the ailing firm, the resulting government bail-out and nationalisation saw the company being renamed to British Leyland. The car manufacturing subsidiary of BL became Leyland Cars, BL Cars Ltd, it was this entity which became Austin Rover. However, the huge industrial relations problems, ineffectual management and product duplication that had plagued the company up to the nationalisation continued throughout the late 1970s; the problems centred on Longbridge union shop steward Derek Robinson. Robinson had assumed a greater level of control over BL than any of its senior managers, his network of union leaders in the various BL plants had the power to end production if he had instructed them to do so; the Labour government of the time ran out of patience with Robinson, appointed South African-born corporate troubleshooter Sir Michael Edwardes to turn BL around. His first task was to curb the large amount of power.
After discovering Robinson's links with various communist groups, the company amassed sufficient evidence claiming that his actions were intended to deliberately damage both BL itself and the UK economy. As a result of this, he was dismissed in 1979. Secondly, Edwardes began a ruthless programme of factory sell-offs; the biggest casualties of this were the MG assembly plant in Abingdon, the Triumph plants in Speke and Canley. BL pulled out of entire markets – for example the large Leyland tractors range was sold off wholesale to Marshall, Jaguar was privatised in 1984. Thirdly, he entered into a collaborative agreement with Honda, the first product of this alliance being the Triumph Acclaim, which paved the way for the joint development of a range of cars which spearheaded the company's revival in the 1980s and 1990s. Lastly, the number of BL dealerships in the UK was trimmed down drastically; the new, slimmer British Leyland was organised into a series of groups. Austin Rover handled the mass production of cars, with the smallest and cheapest models being sold under the Austin brand, while the more upmarket models carried a Rover badge.
High performance version of the Austin hatchbacks and saloons made use of a revived MG badge. Light commercial vehicle production was managed by the Land Rover Group, whilst full-size commercial vehicles were built by Leyland Trucks and Leyland Bus; the luxury manufacturer Jaguar was de-merged from BL in 1984 and privatized taken over by Ford in 1989, was reunited with former BL stablemate Land Rover in 2000 to form what is now Jaguar Land Rover. Sales of Austin Rover products were reasonably strong, though not quite as high as the sales achieved by some of the earlier British Leyland products – the Maestro and Montego for instance did not sell as well as their predecessors, the Austin Allegro and Morris Marina, despite being fundamentally superior vehicles for their time; the Austin/MG Metro was among the top five selling cars in Britain throughout the 1980s, for two years in the early part of the decade it was the best selling supermini in Britain. The Metro, launched in 1980, gave the firm a much-needed competitor in modern supermini market and filled a gap in the range vacated by a scaling down of Mini and Austin Allegro production.
At its peak in 1983, the Metro was Britain's third best selling car with more than 130,000 sales. The Austin Maestro, launched at the beginning of 1983, was very popular, but sales dipped towards the end of the decade and in 1989 it was the 19th best selling new car in the UK with less than 40,000 sales, having peaked in 1984 at more than 80,000 sales as Britain's sixth best selling car; this was less of a problem thanks to the follow-up of the Triumph Acclaim with the first generation Rover 200 of 1984 – the second product of the Honda alliance and one of the few strong-selling small family saloons of its era. So in effect, Austin Rover was selling around 100,000 cars of this size every year in Britain during the mid to late 1980s, regaining its share of the sector after the scaling-down of Austin Allegro production from 1980; the sized Austin Maxi had been discontinued in 1981 to allow the Triumph Acclaim to take over its production lines. The Austin Montego went on sale in April 1984 and sold well, being Britain's seventh best selling car in 1985 and 1986, though it was unable to match the sales success of the sector's established favourites – the Ford Sierra and Vauxhal
Overhead camshaft abbreviated to OHC, is a valvetrain configuration which places the camshaft of an internal combustion engine of the reciprocating type within the cylinder heads and drives the valves or lifters in a more direct manner compared with overhead valves and pushrods. Compared with OHV pushrod systems with the same number of valves, the reciprocating components of the OHC system are fewer and have a lower overall mass. Though the system that drives the camshafts may be more complex, most engine manufacturers accept that added complexity as a trade-off for better engine performance and greater design flexibility; the fundamental reason for the OHC valvetrain is that it offers an increase in the engine's ability to exchange induction and exhaust gases. Another performance advantage is gained as a result of the better optimised port configurations made possible with overhead camshaft designs. With no intrusive pushrods, the overhead camshaft cylinder head design can use straighter ports of more advantageous cross-section and length.
The OHC design allows for higher engine speeds than comparable cam-in-block designs, as a result of having lower valvetrain mass. The higher engine speeds thus allowed increases power output for a given torque output. Disadvantages of the OHC design include the complexity of the camshaft drive, the need to re-time the drive system each time the cylinder head is removed, the accessibility of tappet adjustment if necessary. In earlier OHC systems, including inter-war Morrises and Wolseleys, oil leaks in the lubrication systems were an issue. Single overhead camshaft is a design. In an inline engine, this means there is one camshaft in the head, whilst in an engine with more than one cylinder head, such as a V engine or a horizontally-opposed engine – there are two camshafts, one per cylinder bank. In the SOHC design, the camshaft operates the valves traditionally via a bucket tappet. SOHC cylinder heads are less expensive to manufacture than double overhead camshaft cylinder heads. Timing belt replacement can be easier since there are fewer camshaft drive sprockets that need to be aligned during the replacement procedure.
SOHC designs offer reduced complexity compared with overhead valve designs when used for multivalve cylinder heads, in which each cylinder has more than two valves. An example of an SOHC design using shim and bucket valve adjustment was the engine installed in the Hillman Imp, a small, early-1960s two-door saloon car with a rear-mounted aluminium-alloy engine based on the Coventry Climax FWMA race engines. Exhaust and inlet manifolds were both on the same side of the engine block; this did, offer excellent access to the spark plugs. In the early 1980s, Toyota and Volkswagen Group used a directly actuated SOHC parallel valve configuration with two valves for each cylinder; the Toyota system used hydraulic tappets. The Volkswagen system used bucket tappets with shims for valve-clearance adjustment; the multivalve Sprint version of the Triumph Slant-4 engine used a system where the camshaft was placed directly over the inlet valves, with the same cams that opened the intake valves directly opening the exhaust valves via rocker arms.
Honda used a similar valvetrain system in their motorcycles, using the term "Unicam" for the concept. This system uses one camshaft for each bank of cylinder heads, with the cams operating directly onto the inlet valve, indirectly, through a short rocker arm, on the exhaust valve; this allows a light valvetrain to operate valves in a flat combustion chamber. The Unicam valve train was first used in single cylinder dirt bikes and has been used on the Honda VFR1200 since 2010. A dual overhead camshaft valvetrain layout is characterised by two camshafts located within the cylinder head, one operating the intake valves and the other one operating the exhaust valves; this design reduces valvetrain inertia more than is the case with an SOHC engine, since the rocker arms are reduced in size or eliminated. A DOHC design exhaust valves than in SOHC engines; this can give a less restricted airflow at higher engine speeds. DOHC with a multivalve design allows for the optimum placement of the spark plug, which in turn improves combustion efficiency.
Engines having more than one bank of cylinders with two camshafts in total remain SOHC and "twin cam" unless each cylinder bank has two camshafts. Although the term "twin cam" is used to refer to DOHC engines, it is imprecise, as it includes designs with two block-mounted camshafts. Examples include the Harley-Davidson Twin Cam engine, Riley car engines from 1926 to the mid 1950s, Triumph motorcycle parallel-twins from the 1930s to the 1980s, Indian Chief and Scout V-twins from 1920 to the 1950s; the terms "multivalve" and "DOHC" do not refer to the same thing: not all multivalve engines are DOHC and not all DOHC engines are multivalve. Examples of DOHC engines with two valves per cylinder include the Alfa Romeo Twin Cam engine, the Jaguar XK6 engine and the Lotus Ford Twin Cam engine. Most recent DOHC engines are multivalve, with between five valves per cylinder. More than two overhead camshafts are not known to have been tried in a production engine. However, MotoCzysz has designed a motorcycle engine with a triple overhead camshaft configuration, with the intake ports descending through the cylind